1. Field of the Invention
This invention relates to the field of subsurface exploration and, more particularly, to logging techniques for detecting and locating fractures in earth formations.
2. Description of Related Art
Electromagnetic (EM) logging tools have been employed in the field of subsurface exploration for many years. These logging tools or instruments entail an elongated support equipped with antennas that are operable as sources or sensors. The antennas on these tools are generally formed as loops or coils of conductive wire. In operation, a transmitter antenna is energized by an alternating current to emit EM energy through the borehole fluid (“mud”) and into the surrounding formations. The emitted energy interacts with the borehole and formation to produce signals that are detected and measured by one or more receiver antennas. The detected signals reflect the interaction with the mud and the formation. By processing the detected signal data, a profile of the formation and/or borehole properties is determined.
In conventional EM logging tools, the transmitter and receiver antennas are typically mounted with their axes along, or parallel to, the longitudinal axis of the tool. Thus these instruments are implemented with antennas having longitudinal magnetic dipoles (LMD). An emerging technique in the field of well logging is the use of tools with tilted or transverse antennas, i.e., where the antenna's axis is not parallel to the support axis. These tools are thus implemented with antennas having a transverse or tilted magnetic dipole moment (TMD). The aim of these TMD configurations is to provide EM measurements with directed sensitivity and also sensitivity to the entire conductivity sensor. Logging tools equipped with TMDs are described in U.S. Pat. Nos. 6,044,325, 4,319,191, 5,115,198, 5,508,616, 5,757,191, 5,781,436 and 6,147,496.
A coil or loop-type antenna carrying a current can be approximated as a magnetic dipole having a magnetic moment proportional to the product of the current and the area encompassed by the coil. The direction and strength of the magnetic moment can be represented by a vector perpendicular to the plane of the coil. In the case of more complicated coils, which do not lie in a single plane (e.g. saddle coils as described in published U.S. Patent Application No. 20010004212 A1, published Jun. 21, 2001), the direction of the dipole moment is given by: r×dl and is perpendicular to the effective area of the coil. This integral relates to the standard definition of a magnetic dipole of a circuit. See J. A. Stratton, ELECTROMAGNETIC THEORY, McGraw Hill, New York, 1941, p. 235, FIG. 41. Integration is over the contour that defines the coil, r is the position vector and dl is the differential segment of the contour.
Identification of subsurface fractures is important in oil well exploration and production. Fractures are cracks or breakages within the rocks or formations. Fractures can enhance permeability of rocks or earth formations by connecting pores in the formations. Fractures may be filled with formation fluids, either brine or hydrocarbons. If a fracture is filled with hydrocarbons, it will be less conductive, i.e., a resistive fracture. Deviated wells drilled perpendicularly to resistive fractures tend to be more “productive” (i.e., produce lager quantities of hydrocarbons). Thus, determination of orientations of resistive fractures may help improve gas and oil production. Fractures may be either natural or induced. Natural fractures are those present before the well is drilled, while induced fractures are produced by the drilling process itself. The orientation of a fracture provides the direction of principal stress that affects the stability of the well and it helps in predicting which well trajectory will be the most stable. Knowledge of the presence of and orientation of induced aids in the prediction of fracture strengths of the earth formation. Furthermore, the presence of induced fractures may indicate that the mud weight used for drilling the well is too high so as to cause failure of the rock.
Many methods and systems have been developed for detecting fractures and determining their orientation. For example, U.S. Pat. No. 3,668,619 describes rotating logging tool having a single acoustic transducer that senses the reflected acoustic energy to detect fractures, U.S. Pat. No. 5,121,363 describes a method for locating a subsurface fracture based on an orbital vibrator equipped with two orthogonal motion sensors and an orientation detector. U.S. Pat. No. 4,802,144 uses the measurement of hydraulic impedance to determine fractures. U.S. Pat. No. 2,244,484 measures downhole impedance to locate fractures by determining propagation velocity. Dipole Sonic took are often used to provide fracture orientation, since the presence of fractures produces a velocity difference between soundwaves polarized parallel to the fractures and those polarized perpendicular to them.
There remains a need for improved techniques for detecting and locating fractures, particularly resistive fractures, and for determining their orientations.